This invention relates to a golf ball comprising a solid core, an intermediate layer, and a cover which exhibits improved flight performance, control and feel.
A variety of studies and proposals have been made to find a good compromise between flight distance and control of golf balls. For solid golf balls comprising a solid core and a cover, one common approach is to adjust the hardness and dimensions (including diameter and gage) of the core and the cover.
For example, U.S. Pat. No. 5,439,227 discloses a three-piece golf ball comprising a core, a cover inner layer and a cover outer layer, the cover outer layer being harder than the cover inner layer. U.S. Pat. No. 5,490,674 discloses a three-piece golf ball comprising a solid core of inner and outer layers and a cover, the core inner layer being harder than the core outer layer.
While the respective layers of most golf balls define smooth spherical surfaces, the golf balls disclosed in U.S. Pat. Nos. 2,376,085 and 5,692,973 have a core which is provided with outwardly extending protrusions for preventing the core from being offset during injection molding of the cover therearound. The protrusions in these golf balls are substitutes for the support pins used during injection molding. These patents do not attempt to positively utilize the shape effect of support pin-substituting protrusions, but rather intend to prevent offsetting and to avoid incorporation of a distinct material in the cover. By forming the protrusions from the same material as the cover so that the cover may have a uniform thickness, the protrusions are eventually integrated with the cover. The protrusions themselves do not govern ball performance.
An object of the invention is to provide a golf ball wherein the solid core or the cover partially penetrates into the intermediate layer to form protrusions therein so that the ball is given excellent flight performance, control and feel.
The inventor paid attention to the shape effects of the respective layers constituting a golf ball, especially the buckling phenomenon of protrusions extending from the cover or solid core into the intermediate layer that as the axial compressive load applied to a protrusion increases, uniform compression becomes unstable and is shifted laterally whereby the protrusion is bent. It has been found that a golf ball comprising a solid core, an intermediate layer, and a cover wherein the solid core or the cover is provided with a plurality of protrusions penetrating into the intermediate layer and the protrusions are spaced apart a distance X (mm) from each other and have a length L (mm) is improved in flight performance, control and feel when at least 60% of the protrusions are formed so as to satisfy Lxe2x89xa6Xxe2x89xa65L, and preferably the solid core or the cover that is provided with a plurality of protrusions has a higher hardness than the intermediate layer. The construction where the protrusions of specific spacing-length relationship on the solid core or cover penetrate into the intermediate layer has the following advantages. When the ball is hit with a driver or similar club at a relatively high head speed, the protrusions are bent and the ball is largely deformed, which provides a reduced spin rate and an increased launch angle, resulting in an increased carry. When the ball is hit with a short iron or similar club at a relatively low head speed, the protrusions are not substantially bent and the ball is restrained from deformation, which provides an increased backspin rate and maintains ease of control. With respect to the feel of the ball when hit, the ball gives a soft feel upon driver shots and a tight, full-body, pleasant feel upon short iron shots. In order that the protrusions exert the unique performance as mentioned above when hit with a driver at a relatively high head speed, the protrusions must be arranged at a sufficient spacing to avoid mutual interference so that the protrusions are sufficiently deformable. For at least 60% of the protrusions, their spacing X and length L must satisfy Lxe2x89xa6Xxe2x89xa65L.
Accordingly, the present invention provides a golf ball comprising a solid core, an intermediate layer, and a cover, wherein the solid core or the cover is provided with a plurality of protrusions penetrating into the intermediate layer, the protrusions are spaced apart a distance X (mm) from each other and have a length L (mm), and those protrusions satisfying Lxe2x89xa6Xxe2x89xa65L account for at least 60% of the entire protrusions.
Preferably, the protrusions have a length L of 1 to 6 mm. Also preferably, the protrusions have a maximum size in cross section of 0.5 to 5 mm. The solid core or the cover that is provided with a plurality of protrusions preferably has a higher hardness than the intermediate layer, with a hardness difference being at least 6 Shore D hardness units. Typically, the solid core is formed mainly of 1,4-cis-polybutadiene, the intermediate layer is formed mainly of a polyester elastomer or polyurethane resin, and the cover is formed mainly of an ionomer resin.